In the graphic art field, printing on a printing plate is carried out using one set of color separating film prepared using a lith film from a color original. In general, prior to regular printing (actual printing works), for the sake of checking errors in the color separation step or necessity of color correction, etc., a color proof is prepared from the color separating film. The color proof is desired to realize a high resolving power enabling high reproducibility of intermediate tone images and have such a performance as high step stability. Also, for the sake of a color proof resembling to an actual printed matter, it is preferred to use materials actually used in the printed matters as materials to be used for the color proof, for example, an actual paper stock as a substrate and a pigment as a coloring material. Also, as the method of preparing a color proof, a demand for a developing solution-free dry method is high.
As the dry color proof preparation method, following the recent spread of electronic system in the pre-printing step (pre-press field), a recording system of preparing a color proof directly from digital signals is developed. Such an electronic system is aimed to prepare a color proof having an especially high image quality and reproduces halftone dot images of 150 lines or more per inch. In order to record a high-image quality proof from digital signals, laser beam capable of being modulated by digital signals and of finely limiting recording light is used for a recording head. For this reason, development of an image forming material exhibiting a high recording sensitivity to laser beam and having a high resolving power enabling reproduction of high-definition halftone dots becomes necessary.
As an image forming material to be used in the transferred image forming method utilizing laser beam, a thermofusible transfer sheet comprising a support having thereon a photothermal converting layer of absorbing laser beam to generate heat and an image forming layer having a pigment dispersed in a thermofusible component such as waxes and binders in this order is known (JP-A-5-58045). In the image forming method using such an image forming material, the image forming layer corresponding to a laser beam irradiated region of the photothermal converting layer is melted by the heat generated in that region and transferred onto an image receiving sheet laminated and aligned on the transfer sheet, whereby a transferred image is formed on the image receiving sheet.
Also, JP-A-6-219052 discloses a thermal transfer sheet comprising a support having thereon a photothermal converting layer containing a photothermal converting substance, a thermal release layer that is a very thin layer (from 0.03 to 0.3 μm), and an image forming layer containing a coloring material in this order. In this thermal transfer sheet, a bonding force between the image forming layer and the photothermal converting layer bonded to each other via the foregoing thermal release layer is reduced upon irradiation with laser beam, whereby a high-definition image is formed on an image receiving sheet laminated and aligned on the thermal transfer sheet. The image forming method using the foregoing thermal transfer sheet utilizes so-called “abrasion” and concretely, utilizes a phenomenon in which since a part of the thermal release layer is decomposed and vaporized in a region where laser beam is irradiated, the bonding force between the image forming layer and the thermal conversion layer in that region becomes weak, whereby an image is transferred onto the image receiving sheet laminated on the image forming layer in that region.
These image forming methods have such advantages that an actual paper stock provided with an image receiving layer (adhesive layer) can be used as an image receiving sheet material and that a multicolor image can be easily obtained by transferring images having a different color onto an image receiving sheet one after another. In particular, the image forming method utilizing abrasion has such an advantage that a high-definition image is easily obtained and is useful for preparing a color proof (DDCP: direct digital color proof) or a high-definition mask image.
With the advance of the DTP circumstance, in the use destination of CTP (Computer To Plate), a film take-up step in the middle becomes unnecessary, and the need of proof by means of the DDCP system becomes strong in place of proof printing or proof of analog system. In recent years, a large-size DDCP having higher grade and higher stability and having excellent print conformity is being demanded.
The laser thermal transfer system can undergo photographic printing with high resolution, and there have hitherto been known systems such as (1) a laser sublimation system, (2) a laser abrasion system, and (3) a laser fusion system. However, all of these systems involved such a problem that the recording halftone dot shape is not sharp. The laser sublimation system (1) involved such problems that since it uses a dye as the coloring material, approximation property to a printed matter is not sufficient and that since it is a system in which the coloring material is sublimated, outlines of halftone dots get blurred so that the resolution is not sufficiently high. On the other hand, in the laser abrasion system, approximation property to a printed matter is good because a pigment is used as the coloring material. However, since this system is a system in which the coloring material scatters, it involved such a problem that likewise the sublimation system, outlines of halftone dots get blurred so that the resolution is not sufficiently high. Further, the laser fusion system (3) involved such a problem that since fused materials flow, clear outlines do not appear.
Also, for the sake of shortening the recording time in image recording using laser beam, laser beam made of multiple beams, which use a plurality of laser beams, is recently employed. If recording is performed with laser beam made of multiple beams using a conventional thermal transfer sheet, there may be the case where the image density of a transferred image formed on an image receiving sheet is insufficient. In particular, a reduction of the image density becomes remarkable in the case of performing laser recording with high energy. As a result of investigations made by the present inventor, it was noted that the reduction of the image density is caused by unevenness of transfer generated in the case of laser irradiation with high energy.
Further, there were encountered such problems that the register accuracy is not sufficient and that wrinkles are liable to be generated at the time of actual paper stock transfer.
Also, there was the case of causing such an inconvenience that in accumulating image receiving sheets having an transferred image onto a printed thermal transfer sheet as laminated after image transfer from a variety of thermal transfer sheets to image receiving sheets in a tray, etc., the thermal transfer sheets are dropped from the tray, or the image receiving sheets are curled.
Also, there was encountered such a problem that when the size is made large, traveling property of thermal transfer sheets or image receiving sheets becomes difficult, or jamming or other troubles are generated.
Moreover, there was encountered such a problem that the image quality is reduced by scuffing of the cut surface because of cutting failure of thermal transfer sheets, or foreign matters such as contaminants generated during cutting.